In recent years, following an increase in the degree of density and integration of hybrid ICs and printed circuit boards that carry transistors, ICs, memory elements, and other electronic components, various thermally conductive silicone grease compositions comprising an organopolysiloxane and a thermally conductive filler such as an aluminum oxide powder, zinc oxide powder or the like have been used in order to achieve efficient heat transfer from such devices (see Japanese Unexamined Patent Application Publications (hereinafter referred to as “Kokai”) Sho 50-105573, Sho 51-55870, and Sho 61-157587). However, a problem associated with conventional silicone grease compositions is that a part of the oil bleeds out, and this impairs reliability of the respective electronic devices.
On the other hand, in order to increase content of a thermally conductive filler in a thermally conductive silicone grease composition, it was proposed to use a thermally conductive silicone grease composition comprising an organopolysiloxane, a thermally conductive filler, and an organohydrogenpolysiloxane having in one molecule at least three silicon-bonded hydrogen atoms (see Kokai Hei 4-202496). However, such a thermally conductive silicone grease composition was subject either to oil bleeding or to dripping from thick coating layers or from vertical surfaces under the effect of heat.
It is an object of the present invention to provide a thermally conductive silicone grease composition which is characterized by excellent resistance to heat and reduced oil bleeding.